I am a fan of the spiders. Root the enemy pet and then leech health. As long as you aren't going up against a beast enemy you are bloody hard to kill. Also Legs has a really neat attack where you can do a really strong nuke. Nice to use if you just want to end a fight quickly.
I found Legs to be too weak, tbh. He's an uncommon-quality pet, and I have the ability to use rares in every category. It's possible that his abilities make him worth pursuing, though.
I'll have to give my spider more of a chance; I caught a rare a few days ago.